Nova pivot fields with custom pivot model

I've got a many to many relation that includes some pivot fields that I need to be editable in Nova (3), so I've set it up like this:

            BelongsToMany::make('Orders')
                ->fields(
                    function () {
                        return [
                            Number::make('Quantity')
                                ->default(1)
                                ->min(1),
                            Select::make('Weekday')
                                ->default(Weekdays::monday())
                                ->options(array_combine(Weekdays::toValues(), Weekdays::toLabels()))
                        ];
                    }
                )

However, as you can see, that "weekday" field is an enum (Spatie), and could do with casting, so I'm introducing a custom model for my pivot to apply the cast. From what I've seen in other examples, the code should change to pass a pivot model instance to the fields method:

            BelongsToMany::make('Orders')
                ->fields(new OrderSchedule())

But now it seems that the pivot fields are no longer available – where do I need to put the pivot field definitions for Nova? Do I need to make a separate Nova resource for the pivot model?